The 27-year-old is expected to finalise his club-record switch to Goodison Park in the next 24 hours after successfully passing a medical on Wednesday afternoon.
After apparently agreeing to hand over £45m to land Sigurdsson from the Swans, Koeman insisted that his side are getting good value for their money as the Icelandic playmaker can help fill the void left by Romelu Lukaku.
"We worked a long time to get this deal in. He was one of the key players to bring in," he told reporters. "We knew we would lose Lukaku for this season and then we need player who would bring in productivity to the team.
"We lost 25 goals with Lukaku and we need more players than one to get that number of goals. Gylfi is that type of player with a lot of productivity.
"He knows the Premier League. He had a good season with Swansea and he is, in my opinion, one of the best in the Premier League in his position."
Sigurdsson, who assisted a league-high tally of eight goals direct from dead-ball scenarios in the top flight last term, has had a disrupted pre-season and did not feature in Swansea's meeting with Southampton last weekend.